Combined conductivity and electrochemical impedance spectroscopy study of Na2FeP2O7 cathode material for sodium ion batteries

Na2FeP2O7 is a cathode material for sodium-ion batteries, distinguished from many other types of cathodes by its superb cycling stability. However, its electrical conductivity has not been characterized. Moreover, the electrode kinetics in sodium-ion batteries in general are poorly understood. Both are crucial for further optimization of battery materials and electrodes. In this work, we firstly analyze the electrical conductivity of Na2FeP2O7 along with its activation energy, which is approximately 0.48 eV - comparable to that of Na2MnP2O7 and its lithium analogue Li2FeP2O7. Next, we systematically analyze electrochemical impedance spectra of Na2FeP2O7 electrodes as a function of temperature, electrode thickness, presence of conductive carbon additive and cycling history. We find that a simplified equivalent circuit is sufficient to describe the obtained impedance spectra and analyze the two fitted resistances to attribute the correct interpretation. The results give insight into conductivity of Na2FeP2O7 and help interpret the EIS data obtained for sodium-ion battery electrodes, paving a further way towards electrode optimization.